This invention relates to dynamoelectric machines with distribution of the winding coils for minimizing voltage stresses and the method of locating minimal voltage stress locations.
Various rotating dynamoelectric machines, including alternators and motors, include a power winding forming a part of a stator assembly. The power winding includes a plurality of circumferentially distributed winding coils located within circumferentially distributed winding slots of the annular stator core. In multiple phase machines, the individual phase winding coils are circumferentially displaced relative of all other phase windings. In turn, each individual phase winding will consist of a plurality of coils which are circumferentially distributed within the slots for that phase. Thus, the sides of the individuals coils are generally located in particular slots. However, the coil end turns of the several phase windings project from the opposite ends of the stator core and are consequently located in overlapped and generally engaging relationship. Because of the voltage differences in the windings, special insulation must be interposed between the coils of different phases to electrically isolate the coils belonging to different phases and prevent arcing and shorting of the windings. Traditionally, phase separators, which are also known as phase papers, are interposed between the coils of different phases and particularly of opposite polarity to establish a high level of electrical insulation therebetween. Proper initial locations of the phase insulating separators and maintaining such proper location is required and necessary to achieve and maintain a high level of electrical isolation. A similar situation may exist within the stator slots when multiple layer windings are employed and where the slots are shared by coil sides of significantly different voltage potentials. In this case, slot cell separators are interposed between coils of differing potential. As with phase insulating separators in the coil end turns, the slot cell separators must be installed with skill and care to ensure that stray conductors of a given coil do not breach the separator and make physical contact with the other coil occupying the same slot. Production testing, generally including surge testing and corona testing, is used to determine if the winding insulation does or does not meet predetermined minimum test criteria.
The end turn and intraslot regions of the coils include maximum voltage stress areas. Thus, the actual stress within the end turn and intraslot regions will vary from coil to coil depending upon the voltage differences.
It is well recognized that in addition to being subjected to normal sinusoidal potential stresses arising from the normal voltages within the windings, the end turn and intraslot regions are often subject to abnormal voltage transients, such as lightning strikes and circuit breaker closures. Further, in systems, and particularly motor systems which are connected to and powered from non-sinusoidal supplies, such as pulse width modulated inverters, the end turn and intraslot regions of the coils are further subjected to rather severe stresses due to the inherent transient and non-uniform distribution of voltage within the windings and coils from such supplies. The severity of the non-uniform voltage distribution is a function of the shape of the impinging waveform and the high frequency equivalent circuit parameters of any particular winding.
Repetition rate, or frequency, of such non-uniform voltage distributions is a highly significant, if not a critical factor, in determining the useful life of the particular insulation system within the end turns of the winding. Although the frequency of non-uniform voltage distributions resulting from events such as lightning strikes will generally be on the order of once per year or less, the frequency of such non-uniform voltage distribution may readily be 10,000 Hertz (Hz) or greater with a pulse width modulating inverter supply.
The multi-phase winding of dynamoelectric machines is known to permit some variation in the coil placement for any given phase winding. Thus, the coil windings are not specifically limited to very specific coil slots but rather some limited variation in slot location can be made as well as some variation in the connection of each multi-phase winding to a power supply or the like. Thus, of the plurality of related coils in the given phase which produce an identical polarity and resulting magnetic field, either one of the multiple coils may be chosen for positioning within particular coil slots. Applicant has recognized that by giving due consideration to placement of the coils within and of particular phases and the interconnection to the power supply terminals, the end turn and intraslot voltage stresses created may be of significantly different levels, and further can be determined and an appropriate selection made by appropriate analysis of the different coil physical distributions and electrical connections. To the inventor's knowledge, however, the prior art has not given any programmed consideration in the design of the winding placement which involves the coil slot placement relative to the magnitude of the resulting voltage stresses and has relied on establishing proper insulation separators between adjacent coils of sufficient characteristic to pass the various voltage tests.